The stability of a plane-parallel stationary advective binary mixture flow in a horizontal layer due to horizontal temperature and concentration gradients is investigated theoretically. The layer surfaces are assumed rigid. The main plane-parallel flow and its linear stability against small two-dimensional disturbances is studied with three types of boundary conditions for temperature and concentration : (a) fixed longitudinal temperature and concentration gradients; (b) a constant longitudinal temperature gradient and impermeability; and (c) heat insulation and impermeability. The spectral problem for the amplitudes of normal perturbations is formulated. The case of mechanical equilibrium in the absence of a density gradient is considered. Further, the case of long wave instability for (b) and (c) is studied analytically by an asymptotic expansions technique. The results of numerical simulation of a complete spectral problem for amplitudes are discussed for Pr = 0.01 and Sc greater than or equal to Pr. The limits of the flow stability and the characteristics of critical perturbations are determined for all the indicated types of boundary conditions. The instability is shown to be caused by hydrodynamic and thermoconcentration (double diffusive) mechanisms.